This invention relates to systems and methods used in crude oil production. More specifically, the invention relates to methods for treating the rag mixture that accumulates at the oil/water interface inside of separation, dehydration and desalting equipment.
In the production of heavy crude oils such as bitumen, which typically have an API gravity less than 10, light diluents are used to adjust the API gravity to above 15 API. The combination of diluent and bitumen is commonly referred to as “dilbit.” Bitumen is produced from subterranean formations by a wide variety of production methods such as steam injection, propane injection, and hot water injection. After the bitumen is produced, it is blended with the diluent to increase the API gravity to about 15 to 17 API. Once the API is above 12 API, conventional oilfield equipment can be utilized. This equipment routinely consists of a free water knockout or FWKO, a 2-phase or 3-phase gravity separator, a mechanical or electrostatic oil dehydrator (treater) and, in some cases, an electrostatic desalter. Once the dilbit has been dehydrated and desalted to an acceptable level, the dilbit can be sold to a refinery.
The quality of 15 API dilbit is very poor. These blends typically contain high levels of solids from the subterranean formation and large quantities of asphaltenes formed by the additional diluents. The solids and asphaltenes permit the formation of a mixture of dilbit, water, solids and asphaltenes, which routinely accumulate at the oil/water interface inside of separation, dehydration and desalting equipment. This mixture is commonly referred to as “rag.” If the volume of rag cannot be controlled by the addition of heat, chemicals or electrostatics, then it must be drained from the equipment and processed by an external process.
The volume of rag produced can range from a small volume to several percent of the production stream. Typically, the rag produced is about 1 to 3% of the volume of the produced oil. For example, a Canadian producer might handle 50,000 bpd of bitumen and produce about 500 bpd (barrels per day) of rag.
Additional or external processing of the rag may involve collecting the rag in a large tank to permit separation by gravity. Tank processing, however, requires large volumes of rag and can take days to effectively dehydrate the rag. Handling waste oil volumes in large tanks, therefore, can be expensive and time consuming. Alternatively, the rag may be processed in a flash treater that heats the rag to a temperature above 250° F. and then flashes it to remove the water. Flash treaters, such as those used on Canadian dilbit, leave all the solids, asphaltenes and production salts in the dilbit. These undesirable constituents must then be processed by the refinery. Many refiners have begun to penalize producers for selling flash treated oil.
A need exists for an improved method of treating interface rag.